One example of a conventional avalanche photodiode is shown in FIG. 1. This avalanche photodiode is fabricated by disposing an n.sup.- -type InGaAs light-absorbing layer 2 on an N.sup.+ -type InP substrate 1, disposing an n-type InP multiplication layer 3 on the light-absorbing layer 2, selectively diffusing a p-type impurity from the top surface of the multiplication layer 3 to thereby form a p.sup.+ -type light-receiving region 4.
A voltage is applied from a voltage source 5 between the p.sup.+ -type light-receiving region 4 and the substrate 1 to provide a reverse bias voltage slightly smaller than the voltage which could cause avalanche breakdown of the p.sup.+ -n junction formed between the multiplication layer 3 and the p.sup.+ -type light-receiving region 4, as shown in FIG. 1. In such a reverse bias condition, a depletion region 6 is formed in the multiplication layer 3 and the light-absorbing layer 2. An optical signal incident on the p.sup.+ -type light-receiving region 4 passes through the depletion region 6 to reach the light-absorbing layer 2, where it is absorbed to generate electron-hole pairs. Due to an electric field, holes drift and are injected into the depletion region 6. The injected holes are accelerated in the multiplication layer 3 and collide with lattice atoms in the depletion region 6 with high kinetic energy so that the atoms are ionized to produce electron-hole pairs. The phenomena occur in a concatenating fashion so that a large number of holes and electrons are generated. The thus generated holes flow through the light-receiving region 4 to the voltage source 5. On the other hand, the electrons flow from the depletion region 6 through the light-absorbing layer 2 and the substrate 1 to the voltage source 5. Thus, a multiplied current corresponding to the optical signal incident on the light-receiving region 4 flows.
It has been desired to improve the gain bandwidth product (GB product) of avalanche photodiodes. In order to improve the GB product of an avalanche photodiode such as the one shown in FIG. 1, it is necessary to increase the maximum electric field in the depletion region 6 so that holes can pass through the depletion region 6 as fast as possible, which, in turn, requires that the field gradient in the depletion region 6 be made steep by increasing the carrier concentration in the multiplication layer 3. This results in increase of the maximum field. In order to increase the carrier concentration in the multiplication layer 3, the distance WB (FIG. 1) between the bottom surface of the light-receiving region 4 and the upper surface of the multiplication layer 3 should be made smaller. FIG. 2 illustrates the relationship between the carrier concentration Nb of the multiplication layer 3 and the GB product of the avalanche photodiode, and the relationship between the carrier concentration Nb and the width WB. It is seen that in order to increase the GB product, the carrier concentration must be increased, which requires that the thickness WB must be made smaller. In particular, if the carrier concentration of the multiplication layer 3 is desired to be increased to 2.times.10.sup.17 cm.sup.-3 so as to provide a GB product of 100 or more, the thickness WB must be decreased to a value on the order of 0.2 .mu.m. The precision of the thickness WB is dependent on the precision of the selective diffusion into the p.sup.+ -type light-receiving region 4. For a small thickness WB on the order of 0.2 .mu.m, the selective diffusion of an impurity into the region 4 must be controlled with a precision of the order of 0.1 .mu.m. It has been difficult to control such selective diffusion with such a precision of the order of 0.1 .mu.m
Japanese Unexamined Patent Publication No. HEI 1-261874, for example, discloses a technique for improving the GB product of a photodetector. According to this Japanese patent application, an InP layer and one higher impurity concentration n.sup.+ -type InP layer and one lower impurity concentration n.sup.- -type is disposed immediately beneath a p.sup.+ -type light-receiving region, with the n.sup.- -type InP layer being disposed adjacent to the light-receiving region. The set of these InP layers is used as a multiplication layer with an increased thickness, so as to provide a large GB product. This Japanese application states that the thicknesses of the lower and higher impurity concentration layers are 0.3-0.6 .mu.m, but it is silent about the ratio of the thicknesses. Assuming that the thicknesses of the higher and lower impurity concentration layers are equal to each other, the maximum field in the higher impurity concentration layer is smaller so that sufficient multiplication cannot be expected.
An object of the present invention is to provide an avalanche photodiode free of the above-stated disadvantages, and also a method of making such an improved avalanche photodiode.